Following the Breadcrumbs: How Glacially Deposited Boulders Clue Us into Climate Changes

University of Maine paleoclimatology professor Aaron Putnam extracts a sample from a glacially deposited boulder on the Tibetan plateau.

Throughout our planet’s existence, Earth has experienced periods of warming and cooling, and its glaciers have expanded and receded according to this natural variation in global temperatures. However, our current rate of climate flux is not so typical, and scientists are using glacially deposited boulders to prove it.

"The landscape’s morphology tells a story of our [planet’s] climate history," said Aaron Putnam, a paleoclimatology professor at the University of Maine. Putnam and his team are spending this year’s field season researching boulder deposits along moraines on the Tibetan plateau.

Moraines are hills formed at the point of a glacier's maximum extent; they are the footprints that glaciers of past ice ages left behind. Moraines act as visual timelines for a glacier's history through periods of climate changes. Boulders perched on moraines were deposited there when the climate was stable and the glacier was at an equilibrium, neither expanding or shrinking, whereas boulders scattered up the valley were abandoned as the climate warmed and the melting glacier retreated up the mountain.

By comparing the deposit dates and locations of these glacially abandoned boulders, scientists are able to calculate the rates of recessions of the glacier: the rates of past global warmings. How do scientists calculate such a date as when a glacier dropped a boulder?

Beryllium 10.

A glacial valley on the Tibetan plateau; this summer’s field site for a team of paleo-climate geologists from the University of Maine.

Beryllium 10 is a unique isotope that is collected in the surfaces of the boulders as they are exposed to cosmic rays in our atmosphere. “It starts building and building over the years as an archive—it acts as a cosmic clock,” said Putnam.

The scattered boulders that the scientistssample originated high up in the mountainsas rock debris, which was ripped off themountainside by compacted snow at the topof the glacier. The force of ice sent the rockon a grinding journey down the mountain:shaving it down, rounding its edges and polishing its surface.

After years in frozen darkness, the boulders reemerged from their icy prison fresh-faced and, theoretically, with any beryllium 10 deposits from its past life on a mountainside eliminated. Their reemergence starts a new exposure to cosmic rays, and the beryllium 10 begins collecting like tick marks in the progression of time. “That boulder is capturing the moment that the glacier left it there,” said Putnam.

A researcher clears dust out of holes that were drilled into a boulder as part of the sample removal process.

At Putnam’s field site high up on the Tibetan plateau, the boulders retain roughly 70 atoms of beryllium 10 in every gram of quartz per year—the lower density atmosphere at the high elevation allows for more cosmogenic rays to interact with the boulders than at lower altitudes—and the scientists can use this information to calculate an absolute deposit date for each boulder sample.

By looking into our planet's climate history, scientists gain a better understanding into the effect humans are having on our rapidly warming planet. “We can look at the past and piece together the puzzle of how the climate system works,” said Putnam, “and then from that, be able to predict how perturbing it with greenhouse gasses, the way that we are, may influence change.”